Polymerizable liquids are known to polymerize by the action of heat or light or due to another factor during a production step, storage, or transportation to often create problems. The addition of a polymerization inhibitor such as phenothiazine or a phenol compound, e.g., hydroquinone and derivatives thereof, and contact with an oxygen-containing gas are known to be effective methods for preventing the occurrence of such problems, i.e., for preventing the polymerization of polymerizable compounds. These are techniques in extensive use.
There are cases where a mechanical seal is used as the seal in a pump for liquid transfer.
A mechanical seal comprises a driven ring capable of moving in the axial direction with the wear of the sealing terminal and a sheet ring which does not move. The contact surfaces have been well ground so that a fluid is sealed by the sealing end faces which are perpendicular to the axis and rotate relatively to each other.
A contact pressure is given by the pressure of the fluid to be sealed, a spring, or the pressure of another fluid, or by a rubber, bellows, magnetic force, etc.
There are many kinds of mechanical seals. Although single-mechanical seals are generally used frequently, double-mechanical seals are frequently used for handling fluids such as high-viscosity liquids, high-temperature or low-temperature liquids, and slurries. The mechanical seals are classified by a combination of basic functions as an unbalance type, balance type, rotational type, stationary type, inward flow type, outward flow type, back-end-face high-pressure type, back-end-face low-pressure type, inside-spring type, outside-spring type, etc. Static surface-pressing mechanisms which apply a pressure to the ground surfaces include a coil spring, magnetic force, rubber, metal-welded bellows, and the like.
Mechanical seals employ flushing, quenching, cooling, and other mechanisms for the purposes of cooling and lubrication and preventing foreign particles from accumulating. The flushing mechanism is a mechanism for injecting a liquid (flushing liquid) into the sliding part of a shaft seal part. With this flushing liquid, the mechanical seal is cooled to a temperature in a suitable range. This flushing liquid further functions not only to prevent the high-pressure-side fluid from vaporizing at the sealing end faces to improve lubrication but also to prevent impurities from accumulating in the shaft seal part.
Flushing techniques include self-flushing, in which the liquid itself being transferred is used for flushing, and external flushing, in which a liquid other than the liquid being transferred is injected.
The quenching mechanism is a mechanism for passing a cooling fluid through a non-sliding part of the shaft seal part. The primary purpose of this quenching mechanism is to keep the temperature of the sealing end faces in a suitable range. However, in the case where a volatile fluid, a fluid which is apt to form crystals, a harmful fluid, or the like is handled, the quenching mechanism is frequently used for the purpose of washing off the fluid which has leaked out. Examples of the quenching mechanism include ones employing a seal cover having an auxiliary bushing and ones employing a seal cover having an auxiliary gland.
The cooling mechanism is a mechanism by which a fluid is injected into and discharged from a part other than the sealing end faces in the mechanical seal, and is intended to keep the temperature around the mechanical seal in a suitable range. It is frequently used for cooling mechanical seals which handle high-temperature fluids.
In the mechanical seal part of a pump which transfers a polymerizable liquid, a leakage occurs frequently because the pump contents themselves (e.g., polymerizable liquid) or sludge or a polymer contained in the contents tenaciously adheres to the sliding part (sealing part) or another part or because the contents polymerize due to the heat generated in the mechanical seal part and the resultant polymer tenaciously adheres to the sliding part.
There is a seal utilizing a bellows-shape spring film member or the like (bellows seal) as a seal for preventing a sludge or the like from tenaciously adhering to the sliding part. However, this seal also suffers a leakage in the case of a fluid which polymerizes through a polymerization reaction, because a polymer generates and tenaciously adheres to the sealing surfaces and bellows part to reduce the function of pushing against the sliding part or cause a working failure, and further suffers a leakage due to the breakage caused by the concentration of the rotating force of the pump shaft on the bellows.
A maintenance operation has hence being conducted hitherto in which water, warm water, steam, or nitrogen gas is used to flush or quench the atmospheric side of the sealing surfaces (to conduct insulation from the air, such as cooling, heat insulation, and cleaning). In the case where the liquid to be transferred has a high temperature, a technique is being employed in which a device for cooling the periphery of the sealing region is separately disposed.
Flushing and quenching are effective in preventing adhesion to the sealing surfaces and sliding part of a mechanical seal. However, in the case where the fluid in the pump is one for which water inclusion is not allowable or one which reacts with water or with air contained in water, flushing or quenching cannot prevent the generation of a sludge or polymer although effective in washing off the sludge or polymer which has generated.
Namely, the sealing surfaces of a mechanical seal are lubricated by the formation of a thin film of an enclosed fluid in the space between the contact surfaces of the stationary-side seal face member fixed to the pump main body and the rotational-side seal face member disposed on the rotating shaft, and the enclosed fluid is inhibited from leaking out by the surface tension of the thin film. Since the flushing liquid and the air are in the state of being mixed and contacted with each other at the boundary between these, polymerizable ones undergo a polymerization reaction.